ABSTRACT Having a heart with a steady beat is not optimal. The brain sends signals about various body states including blood pressure and breathing that contribute oscillatory rhythms to the speed of heartbeats and increase heart rate variability (HRV). For reasons that are not yet clear, having greater HRV at rest is associated with better emotion regulation and well-being. People with higher HRV tend to be less anxious, less depressed, less hostile and produce more context-appropriate emotional responses. One potential explanation for the relationship between HRV and emotion regulation is that the same set of brain regions regulates autonomic states and emotions, and so both HRV and emotion regulation reflect the general health and efficacy of this central autonomic network in the brain. However, it appears that the influences do not just flow from the brain to the heart. Paced breathing at the resonance frequency of the heart rate-baroreceptor feedback loop (around 6 breaths/min, a pace often attained during meditative practice) stimulates resonance characteristics of the cardiovascular system and so increases total HRV amplitude dramatically. Recent studies using this approach have shown that increasing HRV during short daily sessions can improve longer-term emotional outcomes. But it is unclear why episodes of high HRV have a positive impact. This project would be the first to examine the brain mechanisms of these effects, testing the hypothesis that episodes of high HRV induced by resonance frequency breathing lead to positive outcomes because they induce dynamic blood flow oscillations in brain regions that monitor and regulate physiological body states. Our experimental manipulation will be a 5-week protocol with random assignment to either a daily session with paced breathing at resonance frequency or one of two control conditions. We test the hypothesis that resonance frequency breathing will enhance measures of emotional well-being, resting state functional connectivity among brain regions involved in emotion regulation, and flexible up- and down-regulation of the amygdala during emotional experience. Furthermore, we test the hypothesis that these outcomes will be mediated by blood flow variability during paced breathing rather than by alternative mechanisms. The expected findings would indicate that HRV is more than just an indicator of health, with an active role in stimulating brain regions in the central autonomic network to improve their coordination and function.